This invention is directed to negative-acting photoimageable compositions which are developable in alkaline aqueous solutions. The invention is particularly applicable to primary photoimaging resists, but is applicable, as well, to compositions that are hardenable so as to form solder masks and the like.
A variety of such photoimageable compositions are described. Essential compositions of the type to which the present invention is directed are A) a binder polymer; B) photopolymerizable .alpha.,.beta.-ethylenically unsaturated compound(s), and C) a photoinitiator chemical system. The binder polymer A) has sufficient acid functionality, generally carboxylic acid functionality, that the binder polymer is soluble in alkaline aqueous solution and thereby renders the photoimageable composition developable in alkaline aqueous solutions. The photopolymerizeable compound(s) B) are monomers and/or short chain oligomers, a substantial portion of which have multiple .alpha.,.beta.-ethylenic unsaturated functionality.
The photoinitiator chemical system C) includes chemicals which generate free radicals upon exposure to actinic radiation. These free radicals propagate the polymerization of the .alpha.,.beta.-ethylenic unsaturated moieties of the photopolymerizeable compounds B). Herein, the photoinitiator system C) is deemed to include not only chemical compounds which generate free radicals, but catalysts or sensitizers which promote the free-radical initiated polymerization of the .alpha., .beta.-ethylenic unsaturated moieties of the photopolymerizeable compounds B).
It was found that when a substantial proportion of the .alpha.,.beta.-ethylenically unsaturated moieties of the photopolymerizeable compounds B) are methacrylic moieties, i.e., at least about 50 mole percent of the .alpha.,.beta.-ethylenic unsaturated moieties of the photopolymerizeable compounds B), that surprisingly high photospeeds were achieved when the photoinitiator system C) included triphenylphosphine at between about 0.005 and about 3 wt % relative to the total weight of A) plus B). While this increase in photospeed is considered highly desirable, other performance characteristics were adversely affected by the addition of triphenylphosphine, including the long-term stability of the system which reduced the useful life of the product and minimized its usefulness.
Also, this type of photoimageable composition typically uses a color former to produce an intense color after exposure. Such a color former produces a contrast between the exposed and unexposed portions of the photoresist. This serves the practical purpose of letting the operator know the resist has been exposed. In addition a recent industry requirement is to try to inspect a panel for obvious defects, such as dirt on the phototool, before any additional processing takes place, so as to avoid the associated costs of material and time of further processing a defective item. Triphenylphosphine, while promoting photospeed, tended to inhibit color formation, thereby reducing contrast and making it more difficult to inspect exposed photoresist layers.
Other properties adversely affected by triphenylphosphine were adhesion (particularly to the copper of a printed circuit board blank) and tenting strength over through-holes.
Accordingly, an investigation was conducted to find a substitute component of the photoinitiator system which would provide high photospeed, yet avoid some of the disadvantages of triphenylphosphine.
Among materials tried as a substitute for triphenylphosphine was n-phenylglycine. N-phenylglycine also provided good photospeed. However, tent strengths were severely reduced and fine line adhesion was very poor. Any reduction in fine line adhesion and tent strength is directly proportional to an increase in circuit board defects.